In U.S. Pat. Nos. 5,485,481 and 5,715,263, which are assigned to the assignee of this patent application and are incorporated herein by reference in their entirety, there is disclosed the utilization of a fiber grating coupled to a gain medium comprising a semiconductor laser source to control, stabilize and maintain stabilization of the operation of the laser within a given wavelength band. The fiber coupled laser is sometimes referred to as pigtailed laser. The assembly of the laser source together with the coupling optics and optic fiber pigtail are provided in a customer-convenient pin-out "package" or industry-standard enclosure that is sealed to hide the components inside the enclosure as well as to improve handling and environmental ruggedness. A few examples of industry-standard enclosures are those known in the industry as "P5", "P6", 14-pin butterfly packages, various coaxial packages such as a TO container and a variety of newer designs including so called MINI-DIL, MINI-SMD and MINI-PIN packages.
In employing a semiconductor laser source having a comparatively wide gain bandwidth, the laser source will tend to have multiple longitudinal modes. Due to changes in various operating conditions, such as changes in ambient temperature or operating current, the laser operation may readily jump from one longitudinal mode to another. Changes in light output intensity caused by the mode jumps are sufficient in magnitude to affect the performance of a fiber amplifier being pumped by such a source because these changes cause a corresponding jump in the reciprocal lifetime of the excited energy level of the amplifier active element, which is usually a rare earth material. This in turn causes noticeable jumps or modulation in the amplified light signal in the amplifier. These modulations cannot be tolerated in optical fiber telecommunication systems.
The foregoing mentioned patents address this problem by placing a fiber grating in the output of the laser source, spaced from the laser source, having a bandwidth sufficiently wide to cause the laser source to operate in the "coherence collapse" regime. In essence, the fiber grating is positioned a sufficient distance from the laser source and the small amount of light reflected back into the laser source can be characterized as a weak source of "noise" to the laser source. The operation of the laser source locks onto this noise-like feedback and maintains its operation within the wavelength bandwidth of the fiber grating. In this coherence collapse regime, the laser source is not allowed to lock onto any one wavelength or longitudinal mode but instead is induced to jump from one longitudinal mode to another. The bandwidth and spacing of the reflective grating is controlled to cause the laser source to jump between modes at a rate that is higher than the reciprocal lifetime of the amplifier active element. In a sense, the amplifier gain element acts as a low-pass filter to smooth the changes in light intensity caused by jumps between longitudinal modes of operation. As a result, mode transitions have little or no effect on the operation of the fiber amplifier.
The length of the pigtail fiber between the laser source output facet and the fiber rating represents an external cavity in addition to the laser source cavity. While coherence is maintained in the laser source cavity, no coherence is established in the external cavity because of its comparatively long length and the wide bandwidth of the grating. To provide reasonable assurance of stabilization in the coherence collapse regime, the length of the pigtail fiber between grating and laser output facet should be greater than the coherence length of the laser source. Also, it is generally preferred that the reflectivity level of the laser source output facet be higher than the reflectivity level of the grating; however, this is not an absolute requirement, depending upon the fiber grating bandwidth and the distance of the fiber grating from the laser source output facet.
The use of a reflector grating in this manner helps bring about coherence collapse because the optical feedback from the fiber grating acts as a perturbation of the coherent optical field formed in the laser source cavity. This perturbation acts to break the coherence of the laser operating mode, which is referred to as coherence collapse, and broadens the bandwidth of the laser emission by several orders of magnitude, resulting in multiple longitudinal mode operation of the laser source. The fiber grating effectively locks the laser source cavity output to the fixed wavelength of the fiber grating and centers the external cavity multi-longitudinal modes around that wavelength. The presence of the multi-longitudinal modes significantly reduces the magnitude of mode-transition noise in the laser so that no single longitudinal mode produced by the laser source contains, for example, more than about 20% of the total optical power produced by the laser source. In addition, the center wavelength of emission remains near the wavelength of maximum reflection from the fiber grating. The laser source is, thus, constrained to operate within the grating bandwidth so that large fluctuations in wavelength of the laser source, such as caused by changes in temperature or operating current, are eliminated. Additionally, the laser source is not perturbed by extraneous optical feedback from reflective components located beyond the fiber grating, provided the level of extraneous feedback is less than that provided by the fiber grating.
An important aspect for achieving coherence collapse is that the fiber grating provides a sufficiently wide bandwidth, such as several GHz, so that no particular longitudinal mode dominates operation of the laser.
The distance between the laser source output facet and the reflective grating is an important consideration in achieving coherence collapse, as mentioned above. For many embodiments, if the grating is placed within a few centimeters or less of the laser source, then the feedback from the fiber grating may be coherent with the optical field inside the laser source cavity and coherent operation of the laser will result. Coherent emission is very useful for some applications but it is much less stable for the application of pumping solid state or fiber amplifiers and lasers because of the mode-transition noise that results when the laser operating characteristics change, such as may result from changes in ambient temperature or operating current. As a result, if the grating is too close to the laser source output facet, intermittent transitions between coherent and coherence collapse states of operation will cause power output fluctuations detrimental to the operation of such amplifiers and lasers.
To assist the maintenance of coherence collapse of the laser emission, the fiber grating should be located at a sufficient optical distance from the output facet of the laser source, which may be, for example, about 50 cm. to 100 cm. from the laser source output facet. This distance should be greater than the coherence length of the laser source so that optical feedback from the fiber grating remains incoherent, thus, helping ensure the laser consistently remains in a state of coherence collapse. The coherence length is related to the bandwidth of the fiber grating in that wider bandwidth gratings can be placed closer to the laser source but it is also related to other parameters such as the operating wavelength of the laser source and the fiber grating reflectivity and period. Beyond the coherence length, the phase of the optical feedback from the fiber grating is uncorrelated to the phase of the laser source emission at substantially all levels of laser current and temperature operation.
The need to locate the fiber grating beyond the coherence length of the laser causes two problems. The first problem is that this distance may impose unacceptable packaging requirements on some applications because a large amount of fiber pigtail is required. On the one hand, if the pigtail is to be included in the same package that contains the laser source, the package must be large enough to contain the length of pigtail fiber required. This is generally impractical for most packages that comprise industry-standard enclosures such as those mentioned above. On the other hand, if a smaller package is used to contain the laser source, the pigtail fiber must extend outside the package. In certain applications, this may expose the pigtail to potential damage as well as to temperature or pressure fluctuations sufficient to adversely affect operating characteristics of the laser source.
The second problem caused by the need to locate the fiber grating beyond the coherence length of the laser source is that longer distances between the grating and the laser source increases the risk that the reflected light may be unable to successfully perturb the laser source and cause frequent mode transitions due to a change in polarization of the reflected light. This risk is significant in embodiments that form the pigtail and reflective grating using ordinary optical fiber rather than more expensive birefringent optical fiber. Unlike birefringent optical fiber, ordinary optical fiber does not maintain the polarization of light. As a result, light propagating along the pigtail fiber from the laser source to the reflective grating experiences a change in polarization, and light reflected from the grating back to the laser source experiences a further change in polarization. If the polarization of the light reflected back into the laser source cavity experiences a total shift approaching ninety degrees, the feedback effects of this reflected light is reduced to essentially nothing. Changes in polarization of this magnitude allow the laser source to operate in a more-or-less free-running state with an uncontrolled wavelength.
One known technique for avoiding this problem is to use birefringent or polarization-maintaining (PM) optical fiber. The use of PM optical fiber is not attractive because it is very expensive compared to the cost of ordinary optical fiber and because it is more difficult to form gratings in PM optical fiber and to align PM optical fiber with other optical components.
A known technique for reducing the risk of this problem is to increase the reflectivity level of the grating formed in the optical fiber. An increase in the reflectivity of the grating is not attractive because it increases the amount of reflected light, thereby reducing the effective output power of the laser source, and because it lowers the level at which jumps between longitudinal modes will occur. Furthermore, as explained above, the level of the grating reflectivity is usually limited by the output facet reflectivity level.
Another known technique for reducing the risk of this problem is to reduce the length of the pigtail fiber but, as explained above, this technique is constrained by the coherence length of the laser source and may result in coherent operation in the external cavity formed between the laser source output facet and the reflective grating.
In view of various considerations such as the problems related to packaging and polarization, it is desirable to locate the fiber grating in the pigtail fiber as close as possible to the laser source. As indicated above, however, stable operation of the amplifier or laser will not be adequately maintained because the laser source will intermittently switch between coherent and coherence collapse states of operation, causing power glitches or power kinks in the amplifier/laser output as operating conditions such as laser operating current and temperature change.